As an internal combustion engine of a vehicle, a high-performance internal combustion engine, including a V-type engine, is sometimes used.
Such a high-performance internal combustion engine is often mounted in a vehicle such that a crankshaft is placed in a direction traverse to a travelling direction of the vehicle (i.e., “transversely placed”). In this case, pistons are arranged at the front and back of the crankshaft.
Accordingly, as shown in FIG. 5, an exhaust device 100 is formed in a shape so as to take exhaust gas thereinto from the front and rear of the internal combustion engine. In this exhaust device 100, an exhaust manifold (exhaust manifold) comprising first catalysts 101 are mounted at the front and rear of the internal combustion engine.
Also, a Y-shaped exhaust junction pipe 103, which collects exhaust gases sent from each of the first catalysts 101, is mounted rearward of the rear-side first catalyst 101. Disposed further rearward of the exhaust junction pipe 103 is a second catalyst 102.
This exhaust junction pipe 103 is mounted rearward of the rear-side first catalyst 101. Because the exhaust junction pipe 103 is away from the front-side first catalyst 101, the exhaust junction pipe 103 and this first catalyst 101 are coupled via an elongated exhaust pipe 104 extending from the exhaust junction pipe 103.
As explained above, in the exhaust device 100 to be coupled to the transversely-placed internal combustion engine, exhaust gas passages from each of the first catalysts 101 to the second catalyst 102 have different lengths from one another. The exhaust pipes 103 and 104 forming such exhaust gas passages are called as an unequal-length exhaust pipe.
In this regard, as shown in FIG. 6, the exhaust junction pipe 103 is formed to be generally Y-shape having three branch parts; the exhaust junction pipe 103 is disposed such that two branch parts are directed to the front side (the internal combustion engine-side) and that the remaining one branch part is directed to the rear side.
Moreover, a cover 105 for inhibiting heat dissipation is attached to the exhaust junction pipe 103 (Patent Document 1).
This cover 105 is very large and covers most of the exhaust junction pipe 103, except for end-side portions of the two branch parts of the three branch parts.
One purpose of inhibiting heat dissipation is, in a case where a catalyst, such as the above-described second catalyst 102 (see FIG. 5), is provided downstream of the exhaust junction pipe 103, to maintain a temperature of exhaust gas flowing into the catalyst at a specified temperature or more, in view of catalytic activity.
For this purpose, as shown in FIG. 5, in the exhaust device 100, a part of the exhaust pipe 104 is formed to be a double-walled pipe structure so as to inhibit heat dissipation.
Moreover, the cover 105 is fixed to the exhaust junction pipe 103 by welding. Specifically, front-side ends of the cover 105, i.e., the ends to be connected to the first catalyst 101 or the exhaust pipe 104, are welded to the branch parts, other than the branch part to which the second catalyst 102 is coupled.
On the other hand, a rear-side end of the cover 105, i.e., the end to be connected to the second catalyst 102, is not welded to the branch part. The reason is as follows: because the cover 105 is fixed at the upstream-side, breakage of the cover 105 may be caused by difference between an amount of heat expansion of the exhaust junction pipe 103 and an amount of heat expansion of the cover 105.
However, if the rear-side end of the cover 105 is not welded, heat may be dissipated from inside of the cover 105; moreover, the exhaust junction pipe 103 and/or the cover 105 may be broken due to mechanical vibrations.
In view of the above, as shown in FIG. 7, the rear-side end of the cover 105 is fixed to the exhaust junction pipe 103 such that a mesh 110 is held between the rear-side end of the cover 105 and the exhaust junction pipe 103.
By holding the mesh 110 therebetween, the mechanical vibrations can be absorbed, and heat dissipation from the rear-side end of the cover 105 can be inhibited.